Recoil-ion momentum distributions for two-photon double ionization of He and Ne by 44 eV free-electron laser radiation

Recoil-ion momentum distributions for two-photon double ionization of He and Ne (ℏω=44 eV) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of ∼1×1014 W/cm2 exploring the dynamics of the two fundamental two-photon–two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models